Method and apparatus of making color cathode ray tube

ABSTRACT

In order to avoid undesirable loss of color disparity in the performance of a cathode ray tube of the multi-beam, multi-color, shadow mask type, it is preferable to limit the diameter of each color dot on the phosphor screen. In the process of fixing phosphor dots on a phosphor screen inside a face plate of the cathode ray tube during the fabrication thereof, a specially shaped light-emitting source is employed, in order to limit the diameter of each phosphor dot to no larger than that of each aperture in the shadow mask through which ultraviolet rays strike the screen to fix said dot. The light-emitting source has a small light-emitting point on the top of the transparent light conduit, which conduit is provided eccentrically on a shaft so that it rotates around in a small circle in a plane parallel with the face plate.

United States Patent 1 9] Hosokoshi 51 Apr. 3,1973

[54] METHOD AND APPARATUS OF MAKING COLOR CATHODE RAY TUBE [75]inventor: Kakulchiro llosokoshi, Neyagawa,

[21] App1.No.: 85,337

[30} Foreign Application Priority Data Oct. 30, 1969 Japan ..44/87363[56] References Cited UNITED STATES PATENTS 3,527,652 9/1970 Sadao Ozakiet al 1 17/335 CM 3,109,116 10/1963 Epstein et a1 ..313/92 B X 3,499,3723/1970 Staunton ..313l92 B X 3,581,136 5/1971 Staunton ..313/92 B XFOREIGN PATENTS OR APPLICATIONS 1,168,826 10/1969 Great Britain.....313/92 Primary Examiner-Alfred L. Leavitt AssistantExaminer-Kenneth P. Glynn Attorney-Craig, Antonelli & Hill [57] ABSTRACTIn order to avoid undesirable loss of color disparity in the performanceof a cathode ray tube of the multibeam, mul'ti-color, shadow mask type,it is preferable to limit the diameter of each color dot on the phosphorscreen. in the process of fixing phosphor dots on a phosphor screeninside a face plate of the cathode ray tube during the fabricationthereof, a specially shaped light-emitting source is employed, in orderto limit the diameter of each phosphor dot to no larger than that ofeach aperture in the shadow mask through which ultraviolet rays strikethe screen to fix said dot. The light-emitting source has a smalllightemitting point on the top of the transparent light conduit,which'conduit is provided eccentrically on a shaft so that it rotatesaround in a small circle in a plane parallel with the face plate.

2 Claims, 4 Drawing Figures METHOD AND APPARATUS OF MAKING COLOR CATHODERAY TUBE This invention relates to an improvement in a method andapparatus for forming phosphor dots on the screen of a color cathode raytube for use in a color television set.

BACKGROUND OF THE INVENTION Hitherto, in a color cathode ray tube of themultibeam, multi-color, shadow mask type, phosphor dots have been fixedin a particular array on the face of the tube by means of thephoto-chemical fixing method, wherein an emulsion containing phosphorand photofixing agent is irradiated by ultraviolet ray beams transmittedthrough apertures in the shadow mask, as explained in US. Pat. No.2,885,935 to D. W. Epstein et al. According to this conventional method,the phosphor dot is inevitably formed with a diameter larger than thatof the aperture in the shadow mask through which ultraviolet rays strikethe screen to fix said dot. Accordingly, the phosphor dots are ofsufficient size to be tangent to each other. On account of this closeproximity of the phosphor dots, even a slight drifting of the path ofthe electron beam may cause undesirable striking of adjacent color dots,resulting in lack of color disparity or inaccurate color expression.

In order to solve this problem, it is preferable to limit the diameterof each color dot so that the color dots are arranged with suitablespaces therebetween. However, such limiting of the diameter of thesevery small dots is very difficult, especially with presently knownmethods of fabrication.

In the conventional method of photo-chemical fixing of phosphor dots, alight source having a hemispherical light-emitting surface is employedto emit photochemical rays, such as ultraviolet rays, onto a screencoated with a phosphor slurry. Details of such process are describedreferring to FIG. land FIG. 2, wherein a phosphor-containing emulsion 9applied on the inside face of the face plate 8 of a color cathode raytube is struck by photo-chemical rays, such as ultraviolet rays 4',which are generated by an ultraviolet lamp 2 in a light box 1. Theultraviolet rays are then transmitted through the transparentlightconduit 3 in the light box 1 and are emitted from the hemisphericalemitting face 4, so as to pass through the correcting lens 5 where theyare suitably refracted. Finally, the rays pass through apertures 7 inthe shadow mask 6, located at a specified distance inside the face plate8, and strike the emulsion 9 in a pattern of dots. Details concerningthe distribution of the ultraviolet rays on the emulsion 9, due to theirpassage through one of said apertures from said light-emitting surface,are shown in FIG. 2, wherein dimensions of parts and their arrangementsas shown are exaggerated for the sake of easy understanding, and thecorrection lens 5 is omitted since it has no substan tial influence overthe distribution of received rays.

As can be understood from FIG. 2, the emulsion 9 is struck by theultraviolet rays 4' coming through each aperture 7, and the energy ofthe received rays is distributed in the form of a circular truncatedcone, which cone is composed of a central part 10 and a surroundingoutskirt part 10', the latter having an outer diameter larger than thatof the aperture 7. The parts of the emulsion 9 thus exposed to theultraviolet rays 4 are hardened by known photo-chemical reaction, andsuch hardnessdepends on the energy of the received rays. Accordingly,the central part of the exposed area acquires the greatest hardness 'andthe hardness gradually diminishes towards the outskirt part of the dotuntil it becomes zero at the outmost boundary.

The exposed emulsion is then developed by being washed with a hot sprayor a shower of washing liquid so as to retain dots at the exposed areas.In order to reduce the diameter of each dot, attempts have been made toremove the portion at the outskirt of each exposed area, where thehardness is comparably low, by applying a rather strong hot shower orspray thereto. However, since the portion at the outskirt of the exposedarea has no critical zone where the hardness of the exposed emulsionchanges abruptly, only the outermost part of the portion at the outskirtcan be removed through the developing process, and in addition, the dotssometimes acquire a non-circular shape during such a process. Thus, ithas hitherto been difficult to obtain phosphor dots with a diametersmaller than that of the apertures on the screen.

SUMMARY OF THE PRESENT INVENTION shaped light emitting source isemployed in order to limit the diameter of each phosphor dot to a sizeno larger than that of each aperture in the mask through whichultraviolet rays strike the screen to fix the phosphor dot. i

BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of thepresent invention will be best understood from the following detaileddescription when read in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an elevation, in section, of a photographic lighthouse havingan optical system for use in making color cathode ray tubes, and ashadow mask and a screen plate of a color cathode ray tube set upthereon;

FIG. 2 is a partially schematic sectional view of the aforementionedprior art optical arrangement including a face plate,phosphor-containing emulsion, shadow mask, hemispherical faced lightsource top, together with a curve showing the distribution of the raysreceived by the emulsion; t

FIG. 3 is a detail view, partly in section, of a lightemitting sourceaccording to the present invention.

FIG. 4 is a partially schematic view of the embodiment of the presentinvention, in section, including a face plate, a phosphor-containingemulsion, a shadow mask and a specially designed light source, togetherwith a curve showing the distribution of the rays received by theemulsion.

In FIGS. 2 and 4, the dimensions of the parts and arrangement thereofare not shown in exact proportions, but are exaggerated for the sake ofeasy understanding, and the correction lens is omitted since it has nosubstantial influence over the distribution of received rays.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 3, the light conduit 11 inthe light box 1 according to the present invention is made of aheat-resistive, transparent material of comparatively high refractiveindex, for example, quartz glass, and is shaped so as to have a smalllight-emitting point 12 on the top thereof and a light-intake face 17 atthe bottom. The light conduit is eccentrically held on a rotating shaft16 which is orientated so as to be substantially parallel with the axisof the face plate 8. Slightly below the top 12, a mask 11' is secured soas to block the ultraviolet rays of the lamp 2 from directly going outof the light box 1. A motor 15 is provided for driving the shaft 16 soas to rotate the light-emitting point 12 in a small circle in a planeparallel with the face plate 8. Similarly to what is shown in FIG. 1, aface plate 8 and a shadow mask 6 are mounted on a light house 1 whichcontains a correction lens and a light box 1. Phosphor-containingemulsion 9 applied on the inside face of the face plate8 is struck byphoto chemical rays, such as ultraviolet rays 12", which are generatedby the lamp 2 in the light box 1, and then are conducted through thelight-conduit 11, so as to be emitted from the light-emitting point 12rotating in the small circle. Next, the rays are suitably refracted bythe correcting lens 5, and finally pass through aperture 7 in the shadowmask 6 located adjacent the face plate 8.

According to the present invention, the photochemical rays, such asultraviolet rays 12', are emitted from the rotating light-emitting point12, as shown in FIG. 4, and the emulsion 9 is struck by the ultravioletrays 12 coming through each aperture 7. Accordingly, when rays emittedfrom the rotating light-emitting point 12 continue to strike theemulsion 9 for a duration of several revolutions of said conduit, theaccumulated energy of the received rays becomes equivalent to the energyemitted from a continuous ring-shaped light source, having a diameterwhich is the same as that of the path of the point 12. Therefore, as canbe understood from FIG. 4, the energy of the received rays isdistributed in a pattern resembling a two-storied circular truncatedcone, which cone is composed of a higher central part 14 and asurrounding lower outskirt part 14'. It is a feature of the presentprocess that there is a considerable difference between the light energylevel of the central part 14 and that of the outskirt part 14',separated by a distinctive circular edge 14" inbetween. The highercentral part 14 has a diameter considerably smaller than that of thelower outskirt part 14'. Moreover, the diameter of the central part 14is made smaller when the diameter of the path of rotation of thelight-emitting point 12 is made larger. The parts of the emulsion 9 thusexposed to the ultraviolet rays 12' are hardened by known photo-chemicalreaction, and such hardness depends on the energy of the received rays.Accordingly, the central part of the exposed area acquires the greaterhardness and the hardness of the outskirt part of the dot is distinctlyless than that of the central area.

The exposed emulsion 9 is then developed by being washed with a hotspray or shower of washing liquid so as to retain dots at the exposedareas. During such development, the less-hardened outskirt part of theexposed dot on the emulsion 9 is selectively removed.

Since the exposed area has a discontinuity at the circular edge 14", theoutskirt part is removed smoothly, and the central part having a higherhardness is retained in a satisfactorily clear circle matching thecircular edge 14". Thus, phosphor dots having a diameter smaller thanthat of the apertures, as well as a smooth round periphery, separated byconsiderable space inbetween, are obtained.

Since the diameter of the phosphor dots can be made smaller than that ofthe aperture of the shadow mask, according to the present invention, itis possible to make the aperture larger than the conventional ones so asto make the picture on the screen face brighter without creating acrowding between adjacent phosphor dots and consequent undesirable lossof color disparity and unwanted mixing of the emitter colors.

While I have shown and described one embodiment in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible of numerous changes and modifications as known to a,person skilled in the art, and I therefore do not wish to be limited tothe details shown and described herein but intend to cover all suchchanges and modifications as are obvious to one of ordinary skill in theart.

What is claimed is:

1. Method for making a color screen for a shadowmask type cathode raytube comprising:

coating one inside face of the cathode ray tube with a photochemicallyhardenable phosphor-containing emulsion,

irradiating said coating through an apertured mask with photochemicalrays emitted from a lamp source, said apertured mask being spaced awayfrom said inside face, rotating said lamp source during said irradiatingin a circle substantially parallel with said coating, wherein thediameter of said circle of rotation is larger than that of the aperturesin said apertured mask so as to create a ray energy distribution patternresembling a two-storied circular truncated cone,

controlling said irradiating and said rotating so as to achieve greaterhardness of the coating areas exposed to the energy represented by theportion above the truncation of said truncated-cone energy distributionpattern than of the coating areas exposed to the energy represented bythe portion below the truncation of said truncated-cone energydistribution pattern, and

washing said irradiated coating to remove all the areas thereof exceptsaid areas exposed to the energy represented by the portion above saidtruncation,

whereby the phosphor dots of the color screen are of a smaller diameterthan the diameter of the apertures of said apertured mask. 9

2. Method as defined in claim 1, wherein said photochemical rays areultraviolet rays.

2. Method as defined in claim 1, wherein said photo-chemical rays are ultraviolet rays. 